The uterus is critically dependent on the hormonal milieu for proliferation and secretory function as well as changes concomitant with implantation and gestation. Of major importance in controlling these events are the ovarian steroids, estrogen and progesterone. Recently, prolactin has been shown to interact with gonadal steroids of both sexes in regulating reproductive tract activities. It is the aim of this research to examine and characterize the interaction between prolactin and estrogen and progesterone in regulating normal uterine activity. Emphasis will be placed on characterization of the uterus as a target organ for prolactin, as well as the effect of prolactin on steroid uptake and binding to uterine cells using mice as experimental models.